The invention relates to image forming devices, such as printers. More particularly, the invention relates to electrophotographic printers including print arrays made up of light emitting elements. The invention also relates to methods and apparatus for compensating for variation in output between light emitting elements in a print array made up of light emitting elements.
Light emitting diode (LED) printbars or print arrays are commonly used in printers because of their high resolution and low cost. They include a large number of closely spaced LEDs arranged in a linear array. An electrostatic image is produced on a recording member by providing relative motion between the LED printbar and a photoreceptor and by selectively providing power to the LEDs at appropriate times. The production of images is performed by having each LED expose a corresponding pixel on the recording member in response to data applied to the printbar through driver circuitry.
Digital data is received from a data source, such as a Raster Input Scanner (RIS), a computer, or a word processor. The digital data is clocked into a shift register. After the start of a line signal, individual LED drive circuits are turned on and off to control the LEDs. The LEDs turned on and off form a line exposure pattern on the surface of the photoreceptor. An image is completed by successive line exposures. Attention is directed to the following U.S. patents, which disclose prior art printhead control circuitry, and which are incorporated herein by reference: U.S. Pat. No. 4,689,694 to Yoshida; U.S. Pat. No. 4,706,130 to Yamakawa; U.S. Pat. No. 5,126,759 to Small et al.; and U.S. Pat. No. 5,138,337 to Ng. Attention is also directed to the following U.S. patents, which disclose prior art exposure control systems, and which are incorporated herein by reference: U.S. Pat. No. 4,525,729 to Agulnek et al., and U.S. Pat. No. 5,025,322 to Ng.
Because the output of each individual LED may vary from that of its neighbors, it is common to compensate for that variation. One method of compensating for variation involves coupling a pulse width modulator (PWM) to each LED. The LEDs are calibrated to produce a uniform output by adjusting the length of the output pulse they produce using the pulse width modulator for each LED. After the LED array has been calibrated, to print a line, signals are sent to the appropriate pulse width modulators to send pulses to their connected LEDs. Different pulse widths are used with different LEDs to compensate for the variation. Other, less accurate, but also less expensive, LED arrays omit the pulse width modulators and drive the LEDs directly. Various systems and methods for achieving LED print bar uniformity are known in the art. Attention is directed to the following U.S. patents, which are incorporated herein by reference: U.S. Pat. No. 5,892,532 to Katakura et al.; U.S. Pat. No. 5,825,399 to Orlicki et al.; U.S. Pat. No. 5,668,587 to Hammond et al.; U.S. Pat. No. 5,666,150 to Ajewole; U.S. Pat. No. 5,250,963 to Smith et al.; U.S. Pat. No. 5,025,322 to Ng; U.S. Pat. No. 4,897,672 to Horiuchi et al.; U.S. Pat. No. 4,831,395 to Pham et al. These patents also provide background information about LED printer hardware. Such hardware could be employed in various alternative embodiments of the apparatus described in the following Detailed Description.
It is also known to use pulse width modulators to enhance resolution in laser printers. See, for example, U.S. Pat. No. 5,134,495 to Frazier et al.; U.S. Pat. No. 5,193,008 to Frazier et al.; and U.S. Pat. No. 5,515,480 to Frazier, all of which are incorporated herein by reference.
LED printers that print with resolutions exceeding LED head resolution are disclosed in U.S. Pat. Nos. 5,648,810; 6,049,349; and 5,751,328 all to Tanuma et al. and all of which are incorporated by reference. These patents disclose setting drive energies for printing drive signals, the drive energies including a first drive energy which is set for actuating LEDs selected in accordance with a first correction output data for printing on basic raster lines and a second drive energy which is set for actuating LEDs selected in accordance with second correction output data for printing on sub-raster lines.
The invention provides a method and apparatus for controlling the outputs of light emitting elements, such as LEDs, in an electrophotographic print array. A predetermined number of pulse width modulators having different width output pulses are used with a greater number of light emitting elements. The pulse width modulators are used, for example, to enhance resolution or so the light emitting elements will produce generally uniform outputs. A multiplexer or logic circuitry is used to select pulse width modulators to be coupled to LEDs.
One aspect of the invention provides a method of controlling light emitting elements in an electrophotographic print head, comprising providing a predetermined number of pulse width modulators having different width output pulses, the predetermined number being less than the number of light emitting elements; selecting multiple light emitting elements for coupling to respective ones of the pulse width modulators; and coupling the multiple selected light emitting elements to the respective pulse width modulators.
Another aspect of the invention provides an electrophotographic printer comprising a print head having a predetermined number of light emitting elements; a predetermined number of pulse width modulators having different width output pulses, the predetermined number being less than the number of light emitting elements; and logic circuitry configured to couple multiple light emitting elements to respective ones of the pulse width modulators.
Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following detailed description, claims, and drawings.